Cytotoxic cells are primary effectors in anti-tumor and anti-viral immune responses. Although the cytotoxic cells, including cytotoxic T lymphocytes (CTL), natural killer (NK) cells, natural killer-like (NK-like) cells and lymphokine activated killer (LAK) cells, are functionally similar, their hematopoietic relationships are not known. The classes of cytotoxic cells may be distinguished by cell surface structures, target specificity and requirements for lymphokines. Our proposal focuses on the immunobioloby of CTL and NK-like cells, and specifically on their development and functional relationship. We and others have demonstrated conversion in vitro of CTL to NK-like cells, a conversion which represents a dramatic alteration in target specificity and perhaps in cell surfaces phenotype. We propose to examine clonal CTL lines and their NK-like derivatives with respect to their requirements in vitro for lymphokines that are essential for proliferation and differentiation. Our preliminary results establish that CTL require multiple lymphokines for proliferation, and we will examine the role of these lymphokines in the expression of cytotoxicity, in the acquisition of the NK-like phenotype, and in proliferation of CTL and NK-like cells. We propose further to examine the molecular nature of the antigen recognition structure responsible for the broad target specificity of Nk-like cells. The T cell receptor which mediates target recognition by CTL has been described; however, what role this structure may play in target recognition by NK-like cells is unknown. Since our system involves the differentiation in vitro of CTL to NK-like cells, we will examine the possibility that the T cell receptor, or a modification thereof, serves as the antigen recognition structure of NK-like derivatives. Alternatively, a new receptor expressed uniquely on NK-like cells may be responsible for target recognition. Such a structure will be identified by the selection of gene sequences expressed specifically in NK-like cells. We will also prepare monoclonal antibodies which react with the surface of NK-like cells, but not with the parental CTL lines. These antibodies will facilitate identification of the antigen receptor on NK-like cells, and may also identify accessory proteins and lymphokine receptors required for NK-like function. In summary, we propose a broad characterization of the immunobiology of CTL and NK-like cells, including an investigation of the lymphokines which regulate their growth and differentiation, and a dissection of the molecular apparatus required for their function.